These screencasts walk you through the process of creating an Overtone development environment.
This episode shows you how to create a project that uses the latest bleeding edge github source rather than a specific clojar. This will allow you to hack about in the Overtone source and see your modifications manifested in a test app.
If any of this walkthrough isn't clear, please comment with questions and I'll try my best to clarify. I'm still very new to screencast production so there's certainly a lot of room for improvement :-)